Amorphous Magnets Research Articles

Wide web coating of complex materials

For over 25 years, the thermal evaporation of aluminium onto thin polymeric webs, such as polyester (PET) and polypropylene (PP), has generated large volumes of barrier packaging films, decorative films, capacitor films and some window films. During the 1980s, the experience of wide web handling was combined with deposition technologies, such as electron-beam evaporation, magnetron sputtering and plasma-enhanced chemical vapour deposition (PECVD), to create a large number of new, exciting, coating materials, including oxides and nitrides of most elements. More particularly, combinations of these coating layers into a complex coating stack led to new products, such as low emissivity, solar heat reflecting, architectural glazing films, electrochromic devices and high performance optical reflectors. With these technologies, unique coating characteristics can be realized, e.g. transparent electrodes, transparent, flexible glassy barriers for moisture and gases and amorphous soft magnetics for security devices. Having their beginnings in exclusive markets, these new, exciting products are now finding their way into larger consumer markets.

Wide web coating of complex materials

For over 25 years, the thermal evaporation of aluminium onto thin polymeric webs, such as polyester (PET) and polypropylene (PP), has generated large volumes of barrier packaging films, decorative films, capacitor films and some window films. During the 1980s, the experience of wide web handling was combined with deposition technologies, such as electron-beam evaporation, magnetron sputtering and plasma-enhanced chemical vapour deposition (PECVD), to create a large number of new, exciting, coating materials, including oxides and nitrides of most elements. More particularly, combinations of these coating layers into a complex coating stack led to new products, such as low emissivity, solar heat reflecting, architectural glazing films, electrochromic devices and high performance optical reflectors. With these technologies, unique coating characteristics can be realized, e.g. transparent electrodes, transparent, flexible glassy barriers for moisture and gases and amorphous soft magnetics for security devices. Having their beginnings in exclusive markets, these new, exciting products are now finding their way into larger consumer markets.

Improved theory of amorphous metallic magnetism and application to amorphous Fe

Abstract A finite-temperature theory of amorphous metallic magnetism has been improved taking into account the amplitude fluctuations of local magnetic moments via variable local band widths. The theory leads to the existence of spin glass (SG) around amorphous Fe, narrows the reentrant SG region, and lowers the Curie temperatures.

T- V magnetic phase diagrams of amorphous Fe

Volume dependence of magnetization in amorphous Fe has been investigated on the basis of the finite-temperature theory of amorphous metallic magnetism. The obtained magnetic phase diagram on the T-V plane shows the existence of spin glass in a wide range of volume, which reasonably explains the spin glass found in Fe-rich alloys.

The magnetic correlation range in speromagnetic Dy7−xYxNi3

The spin correlation range ξ in the amorphous random anisotropy magnets Dy7−xYxNi3 has been measured by small-angle neutron scattering. ξ is typically a few atomic spacings, as expected for a speromagnet. We suggest that previous estimates of ξ (∼ 15 nm), consistent with correlated speromagnetism, may have resulted from magnetic scattering associated with microcrystalline impurities in the amorphous matrix.

Interaction of Alpha Particle Beams with Fe-Based and FeNi-Based Glassy Ferromagnets

AbstractSamples of Fe78B13Si9 and Fe40Ni38Mo4B18 metallic glasses were irradiated with alpha particle beams (W=2.8 MeV) using radiation doses of 1016 and 1017 cm-2. Irradiation-induced effects on the magnetic texture and phase composition of alloy samples were studied by Mössbauer spectroscopy. Related morphological changes and resultant crystalline precipitates were characterized by scanning electron microscopy. The evolution of phases and microstructure during the radiation-induced amorphous-to-crystalline transformation was found to depend on the particle flux and sample composition. The lowest radiation dose employed was found to be more effective in inducing amorphous-to-crystalline transformations in both ferromagnetic alloys studied. In addition, the FeNi-based amorphous system investigated was found to be more stable than the Fe-based metallic glass, exposed to the same particle-beam irradiation conditions. By stimulating unconventional pathways for the crystallization process, the interaction of alpha particle beams with glassy ferromagnets offers unique opportunities to understand the fundamentals of nucleation and growth in amorphous magnets.

The Mössbauer effect and magnetism

The Mossbauer effect enabled the magnetic hyperfine splitting (hfs) in ferromagnetic solids to be observed directly for the first time. It was quickly extended to measurements on antiferromagnets, ferrimagnets and paramagnets, and is now well established as a probe for the study of magnetic materials. Applications have ranged from the “old” problem of the state of the iron atoms in ferromagnetic alloys to the “new” magnetic materials, e.g. amorphous magnets, spin glasses, fine particle magnets and multilayers. Some examples where the Mossbauer effect has made an important contribution are described.

Theory of amorphous metallic magnetism with self-consistent amplitude fluctuations and its application to amorphous Fe.

A finite-temperature theory of amorphous metallic magnetism has been proposed on the basis of the distribution-function method and the functional-integral technique. It improves the previous theory, and describes self-consistently the amplitude fluctuations of local magnetic moments on the nearest-neighbor shell by means of the distribution function of two variables, the local moment, and the local bandwidth parameter on the same site. Application of the theory to amorphous transition metals around Fe shows that the improved theory reduces the ferromagnetism, smoothes the d-electron number dependence of the spin-glass order parameter, and narrows the reentrant spin-glass region. Qualitative features of the magnetic phase diagram, however, is shown to be unchanged. The present theory supports the existence of spin glass in amorphous Fe.

Existence of the spin-glass state in amorphous Fe.

The volume dependence of magnetic properties in amorphous Fe has been calculated on the basis of the finite-temperature theory of amorphous metallic magnetism which self-consistently determines the distribution of local magnetic moments. The calculated magnetic phase diagram on the T-V plane and magnetization-vs-volume curve show the existence of the spin-glass state in a wide range of volume (10.50 A${\mathrm{\r{}}}^{3}$\ensuremath{\lesssim}V\ensuremath{\lesssim}12.5 A${\mathrm{\r{}}}^{3}$) after disappearance of ferromagnetism. The results verify our previous conclusion of the spin-glass state in amorphous Fe, which is expected from the experimental data of Fe-rich amorphous alloys containing early transition metals and rare-earth metals, but disagree with those obtained from the supercell approaches in the ground-state electronic structure calculations. It is shown that the nonlinear magnetic couplings between the nearest-neighbor Fe local moments and the local environment effects on the amplitude of Fe local moments via structural disorder lead to the spin-glass state in the weak magnetic region, while the volume expansion develops the ferromagnetic couplings and therefore the ferromagnetism.

Magnetic properties of a new amorphous magnet

An amorphous phase of Co3BTCA2(H2O)6 has been prepared (BTCA=1,3,5-benzenetri- carboxylate). The static properties (dc susceptibility, hysteresis) of Co3BTCA2(H2O)6 are similar to those of other ‘‘glassy’’ magnetic systems, but the dynamics of Co3BTCA2(H2O)6 (absence of long relaxation times, disappearance of frequency dependence at low temperatures) are unusual. The amorphous phase is characterized by a narrow hysteresis loop at low temperature. For the hysteresis curve at 4.2 K, the coercive field is several hundred Oersted and the remnant magnetization is on the order of 10−2 Msat. The temperature dependence of the dc magnetic susceptibility shows an unusual field dependence below 30 K, with the largest effective moments being obtained in the smallest applied fields. The freezing temperature, Tf, in zero field, as determined by both dc and ac susceptibilities, has been found to be Tf=23 K. The ac susceptibility shows the characteristic spin glass frequency dependence, yet loses all frequency dependence for temperatures below 9 K.

Amorphous magnetism in Fe-B alloys: First-principles spin-polarized electronic-structure calculations.

We present self-consistent spin-polarized electronic-structure calculations for amorphous ${\mathrm{Fe}}_{\mathit{x}}$${\mathrm{B}}_{100\mathrm{\ensuremath{-}}\mathit{x}}$ (50\ensuremath{\le}x\ensuremath{\le}95) alloys, based on a supercell linear-muffin-tin-orbital technique and realistic structure models produced by molecular-dynamics simulations. We show that the electronic densities of states of the amorphous alloys are very similar to that of the crystalline Fe borides. This confirms the conclusions as to the similarity of the local order in the crystalline and amorphous phases drawn on the basis of the structural studies. The calculated composition dependence of the magnetic moments is in good agreement with experimental data on amorphous films, with a maximum of the magnetic moment close to 15 at. % B. The decrease of the moment for lower B content is shown to arise from the competition between ferro- and antiferromagnetic exchange interactions, the decrease with higher B content is due to a dilution effect: polarizable d-d bonds are replaced by magnetically inert p-d bonds. We also present detailed calculations of the photoemission intensities.

Magnetic ordering in amorphous oxides

The magnetic structure of radio frequency sputtered amorphous oxides is investigated by magnetization and ferromagnetic resonance techniques. Samples of Bi/sub 2/O/sub 3/-Fe/sub 2/O/sub 3/ substituted with ABO/sub 3/ (AB=BaTi, PbTi, PbZr) and MeO (M=Cu,Ca,Zn)-type compounds are studied. As-deposited samples are paramagnetic, and samples annealed at high temperatures show a ferrimagnetic character possibly due to the formation of magnetically ordered clusters. Samples with nonmagnetic ABO/sub 3/ show a record room temperature magnetization. Films containing oxides of zinc, copper, or calcium are anisotropic amorphous magnets with a large uniaxial or planar anisotropy. The nature and composition of ordered clusters in some samples are inferred from magnetic characterization of the film. A similar ferrimagnetic character is observed in amorphous samples sputtered at high temperatures. >

Amorphous magnetism in iron-boron systems: First-principles real-space tight-binding LMTO study.

We have performed ab initio spin-polarized self-consistent calculations of the electronic structure and magnetic moments in the series of the ${\mathrm{Fe}}_{100\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{B}}_{\mathit{x}}$ glasses (0x60) with the real-space tight-binding LMTO method. Realistic atomic models consisting of 500 to 864 atoms in a cubic box with periodic boundary conditions were constructed with the use of the Monte Carlo method. The systems studied show itinerant magnetism with the net magnetic moment on iron sites, \ensuremath{\mu}\ifmmode\bar\else\textasciimacron\fi{}, saturating at the maximum value in a region of a (hypothetical) amorphous iron. The iron magnetic moment decreases upon dilution with boron atoms with no maximum on the compositional dependence of \ensuremath{\mu}\ifmmode\bar\else\textasciimacron\fi{}. The calculated distribution of the iron moments is narrow with the width decreasing towards the boron-rich end of the series. The magnetovolume effect is shown to be noticeable in iron-rich glasses, declining with increasing boron content. No evidence of a tendency to form the spin-glass state has been found in iron-rich borides.

A Contribution to the Mean‐Field Analysis of Amorphous Magnets

AbstractA method is outlined to obtain explicit analytical solutions for Handrich's mean‐field model of amorphous ferromagnets for the general spin quantum number S and the structural disorder parameter δ. The explicit solutions are given for S = 1/2 and δ = 1/3 and 1/2.

Hyperfine field distribution, invar anomalies in amorphous FeZr alloys, and problems of amorphous Fe magnetism

The temperature and concentration dependence of the hyperfine field distribution in the Mössbauer spectra of Fe nuclei in FeZr amorphous alloys are studied. The relation between the low-field component and the Invar anomaly is also investigated. The nonmagnetic densities of electronic states of the model clusters Fe14 and Fe14Zr and the influence of cluster deformations simulating the interatomic distance fluctuations are calculated. The results show that the Fe atom with unstable magnetic moment causes the magneto-volume anomaly. The Fe atoms with next-nearest-neighbour Zr atoms possess unstable moments. Some Fe atoms among nearest neighbours also show a decrease of the moments with increasing temperature. These atoms have smaller interatomic distances as confirmed by the density of states calculations. [Russian Text Ignore]

Quantum relaxation in random magnets.

We present a comprehensive study of the low-temperature magnetic relaxation in random magnets. The first part of the paper contains theoretical analysis of the expected features of the relaxation, based upon current theories of quantum tunneling of magnetization. Models of tunneling, dissipation, the crossover from the thermal to the quantum regime, and the effect of barrier distribution on the relaxation rate are discussed. It is argued that relaxation-type experiments are ideally suited for the observation of magnetic tunneling, since they automatically provide the condition of very low barriers. The second part of the paper contains experimental results on transition-metal--rare-earth amorphous magnets. Structural and magnetic characterization of materials is presented. The temperature and field dependence of the magnetic relaxation is studied. Our key observation is a nonthermal character of the relaxation below a few kelvin. The observed features are in agreement with theoretical suggestions on quantum tunneling of magnetization.

Magnetism in amorphous transition metals. II.

Susceptibilities and pressure effects on the magnetic properties of amorphous transition metals have been investigated on the basis of a finite-temperature theory of local-environment effects in amorphous metallic magnetism. Calculated high-field susceptibilities, paramagnetic susceptibilities, effective Bohr-magneton numbers, and the pressure dependence of magnetization, Curie temperature, and spin-glass temperatures are shown to explain various aspects of the magnetism in amorphous and liquid transition metals and alloys as a function of the d electron number. It is demonstrated that these quantities are governed by the electronic structure of amorphous transition metals, in particular, by the main-peak position in the noninteracting density of states for amorphous structure. Moreover, the detailed investigations for susceptibilities, forced-volume magnetostriction, and the T-P phase diagram in the reentrant spin-glass region reveal that the itinerant-electron spin glasses in the Fe-rich amorphous alloys with more than 90 at. % Fe are caused by structural disorder instead of configurational disorder.

ANOMALOUS MAGNETIC BEHAVIORS OF RE-ENTRANT SPIN GLASSES IN AMORPHOUS TRANSITION METALS

The reentrant spin glasses (RSG) in amorphous transition metals are examined on the basis of the finite-temperature theory of amorphous metallic magnetism. It is shown that a competition between the short-range ferromagnetic couplings and the long-range antiferromagnetic couplings forms the itinerant-electron RSG via the structural disorder characteristic to the amorphous pure metals. Calculated high-field susceptibilities, forced volume magnetostriction, and T-p phase diagram explain well the experimental data of Fe-rich amorphous alloys around 90 at.% Fe. This supports that the RSG around 90 at.% Fe in Fe-rich amorphous alloys are dominated by the structural disorder.

On the anisotropy of soft amorphous magnetic alloys

Introduction Despite the absence of any structural long range order, amorphous magnetic alloys can exhibit uniaxial macroscopic magnetic anisotropy. Ferromagnetic states are caused primarily by the exchange interaction among spins of magnetic atoms in crystalline and amorphous magnets. This exchange interaction is isotropic in most eases and therefore does not determine the direction of aligned spin moments with respect to the symmetry axis. For amorphous magnetic alloy, local magnetic anisotropy can be easily averaged out on a macroscopic scale like the magnetic anisotropy in crystals[l]. Magnetic anisotropy energy, the part of free energy which depends upon the direction of aligned magnetic moments is, in ordinary cases, small compared with the exchange energy, so that the magnitude of the spontaneous moment is essentially independent of its direction. In such a case, magnetic anisotropy energy can be defined unambiguously[2].

Thermodynamic reciprocity of magnetic anisotropy and deformation induced by thermoannealing

A phenomenological theory of induced anisotropy which applies for magnets with internal freedom degrees in atom structure has been suggested. This theory is formulated for amorphous magnets and shows the thermodynamic reciprocity of magnetic anisotropy and deformation induced by thermoannealing.